It has recently been proposed to mount one or more sails on an engine-driven ship for effectively utilizing wind force on a voyage of the ship with a view to saving energy. A conceivable sail to be mounted on a ship is the conventional sail made of canvas fitted to the mast. Such a sail requires much time and labor for spreading and furling the sail.
To avoid this inconvenience, a sail comprising thin metal plates or synthetic resin plates applied to reinforced rib members (hereinafter simply referred to as a rigid sail) was invented to take the place of the above-mentioned canvas sail. Since this rigid sail can automatically be opened and closed, it permits solving the above-mentioned problem.
As shown in FIG. 1, the present inventors have previously proposed the following sail as one of the above-mentioned rigid sails, the prior sail being disclosed in Japanese Patent Provisional Publication No. 47,994/80 dated Apr. 5, 1980, hereinafter referred to as "prior art". Said prior art sail comprises:
a mast 2 vertically fitted through a mast support (not shown) onto the deck of a ship 1; said vertical mast 2 being rotatable around the axis thereof;
a rigid sail 4 fitted to said mast 2 in parallel with the axis thereof; said rigid sail 4 comprising a central sail portion 4A fixed through a fixture 3 to said mast 2, and a left sail portion 4B and a right sail portion 4C both fixed to the respective side edges of said sail portion 4A so as to permit opening and closing motions of the rigid sail portions 4B and 4C;
cylinders 5A and 5B for opening and closing said left sail portion 4B and said right sail portion 4C;
whereby said sail portions 4B and 4C are opened and closed relative to said mast by actuating said cylinders 5A and 5B.
However, since the rigid sail in the above-mentioned prior art is folded to form a triangular cross-sectional shape, the rigid sail may create a high wind resistance on a voyage under calm conditions, even when in its folded state.